1. Field of the Invention
The present invention relates to an optical device including a multilayer reflector.
2. Description of the Related Art
(Vertical Cavity Surface Emitting Laser)
A vertical cavity surface emitting laser (VCSEL) which is an optical device can emit light in a direction perpendicular to a semiconductor substrate, so a two-dimensional array can be easily formed. When parallel processing of multibeam emitted from the two-dimensional array is performed, higher density and higher speed can be obtained, so various industrial applications are expected. For example, when a vertical cavity surface emitting laser array is used as an exposure source of an electrophotographic printer, the printing speed can be increased by parallel processing of a printing process using the multibeam.
A vertical cavity surface emitting laser which is currently in practical use is a device for mainly generating laser light of an infrared region (0.75 μm to 0.85 μm). A beam spot can be further reduced as an oscillation wavelength is shortened from the infrared region to a red region, a blue region, and an ultraviolet region, so higher resolution can be obtained. Therefore, the practical use of a vertical cavity surface emitting laser in the regions from red to ultraviolet is required.
There is a great effect attained by a combination of the increase in resolution which is obtained by the shortened wavelength and the parallel processing using the multibeam, its contribution to various fields including applications for a printer is expected. When a vertical cavity surface emitting laser capable of oscillating in a band of from 1.3 μm to 1.5 μm in which dispersion or absorption in an optical fiber is small can be in practical use, long-distance large-capacity communications can be performed using an arrayed fiber and an arrayed optical source.
(Multilayer Reflector)
The feature of the vertical cavity surface emitting laser is to include a cavity provided in a direction perpendicular to an in-plane direction of a substrate. In order to realize a surface-emitting laser which can continuously operate at a room temperature, a reflector whose reflectivity is 99% or more is necessary.
An example of such a reflector to be used includes a multilayer reflector in which two materials whose refractive indices are different from each other are alternately laminated plural times by an optical thickness of λ/4. Here, λ denotes a wavelength of light emitted from an optical device. The optical thickness is obtained by multiplying a thickness of a layer by a refractive index of a material of the layer.
(Near-Infrared Vertical Cavity Surface Emitting Laser)
For a near-infrared vertical cavity surface emitting laser using a GaAs semiconductor which is already in a practical use, a semiconductor multilayer mirror in which GaAs and AlAs which have extremely high crystallinity are combined is used. In addition, a semiconductor multilayer mirror in which AlGaAs whose Al composition is small and AlGaAs whose Al composition is large are combined for constituent layers is used.
However, a long-wavelength (1.3 μm to 1.5 μm) laser for communication and a red (0.62 μm to 0.7 μm) laser have a problem in that their thermal characteristics are undesirable or high-power output is difficult to be achieved.
That is, in an active layer for generating light of the communication wavelength region or the red region, there is no clad layer material capable of sufficiently confining electrons in the active layer at a high temperature ranging from 60° C. to 80° C. Therefore, with an increase in temperature, a large number of electrons are overflowing from the active layer, so the thermal characteristics deteriorate and the high-power output is difficult to perform.
In the vertical cavity surface emitting laser, heat generated by the active layer is confined to the vicinities of the active layer by the semiconductor multilayer reflector whose thermal resistance is high.
Therefore, unfortunately, the vertical cavity surface emitting lasers using the material for generating the light of the above-mentioned wavelength are devices whose temperature characteristics are undesirable.
To be specific, a normal multilayer reflector of the long-wavelength vertical cavity surface emitting laser has a structure in which an InGaAsP layer (high refractive index layer) whose optical thickness is λ/4 and an InP layer (low refractive index layer) whose optical thickness is λ/4 are alternately laminated as a large number of pairs. In this case, the thermal resistance of the InGaAsP layer used as the high refractive index layer is approximately 20 times larger than the thermal resistance of the InP layer used as the low refractive index layer.
In the circumstances, in 47th Seiken Symposium Preprints, pp. 80-81, March 2006 (Precision and Intelligence Laboratory, Tokyo Institute of Technology), a multilayer reflector in which the optical thickness of a multilayer reflector constituent layer is not set to λ/4 is discussed.
To be specific, with respect to the multilayer reflector of the long-wavelength vertical cavity surface emitting laser, in 47th Seiken Symposium Preprints, pp. 80-81, March 2006 (Precision and Intelligence Laboratory, Tokyo Institute of Technology), a multilayer reflector in which the optical thickness of an InP layer whose thermal resistance is small is set to a value larger than λ/4 and the optical thickness of an InGaAsP layer whose thermal resistance is large is set to a value smaller than λ/4 is disclosed in order to reduce the thermal resistance.
The total layer thickness (one pair) of the high refractive index layer and the low refractive index layer which constitute the multilayer reflector is fixed to be an optical thickness of λ/2. Therefore, it is considered that a heat dissipation effect can be improved and thus it is possible to provide the multilayer reflector in which an increase in device temperature can be prevented.
(Ultraviolet/Blue Vertical Cavity Surface Emitting Laser)
A GaN semiconductor material is used for a vertical cavity surface emitting laser in an ultraviolet/blue region (300 μm to 500 μm). For a multilayer reflector, for example, a pair of a GaN material and an AlN material with a relatively large refractive index difference therebetween is selected.
However, the multilayer reflector made of both of these materials has a large lattice mismatch. When several ten pairs are grown by an optical thickness of λ/4, it is more likely to introduce lattice strains into the multilayer by the lattice mismatch. As a result, a crack occurs, so it is difficult to form such a multilayer reflector as to achieve a reflectance of 99% or more.
Therefore, also in Japanese Patent Application Laid-Open No. 2003-107241, a multilayer reflector in which the optical thickness of constituent layers of the multilayer reflector is not set to λ/4 is discussed.
To be specific, a multilayer reflector is disclosed in which the optical thickness of a GaN layer whose thermal expansion coefficient difference with respect to a substrate is small is set to a value larger than λ/4 and the optical thickness of a Al0.6Ga0.4N layer whose thermal expansion coefficient difference with respect to the substrate is large is set to a value smaller than λ/4.
The total optical thickness (of one pair) of the high refractive index layer and the low refractive index layer which constitute the multilayer reflector is fixed to be an optical thickness of λ/2. Therefore, it is considered that the multilayer reflector with few cracks can be provided.
In 47th Seiken Symposium Preprints, pp. 80-81, March 2006 (Precision and Intelligence Laboratory, Tokyo Institute of Technology) and Japanese Patent Application Laid-Open No. 2003-107241, the multilayer reflector including a layer whose optical thickness is not λ/4 is described. However, a design guideline for actually incorporating the multilayer reflector into a cavity is not described therein.
The inventor of the present invention arranged the layers whose optical thickness is not λ/4 as described in 47th Seiken Symposium Preprints, pp. 80-81, March 2006 (Precision and Intelligence Laboratory, Tokyo Institute of Technology) and studied in view of a cavity structure. As a result, it was found that the arrangement causes a deviation of a design value from a resonance wavelength and a reduction in reflectivity.
That is, when the arrangement of the high refractive index layer and the low refractive index layer with respect to the internal optical intensity distribution is not taken into account, it is likely to cause a reduction in yield due to the deviation of the resonance frequency or deterioration of device characteristics due to a reduction in reflectivity. As a result, although the intended use of the layer whose optical thickness is not λ/4 is made for improving the characteristics, a problem that the effect of such use cannot be obtained occurs.